The reliable operation of controlled resistive loads is critical to many control system applications, particularly in temperature control systems using resistance heating. For example, the failure of resistance heating cables used for freeze protection in a wet sprinkler system can result in collateral water damage due to frozen water pipes, as well as, in the failure of the sprinkler system to function when needed. In many applications, such resistance heaters can be inactive for extended periods of time. Consequently, heater failures must be discovered before operation of the heater is required. Generally in control system topology, a resistance heater or other resistive load is connected to a line voltage power source through an electric controller, which controls the line voltage applied to the load. The controller opens and closes the contact switch to operate the heater or load as needed.
Electrical controllers have been developed which include monitoring circuits for detecting open circuit failures in resistive loads while the loads are under power. Generally, the failure detection circuits in the controllers test a load by monitoring an actual operational current flow through the resistive load. Consequently, the loads must be periodically operated during idle conditions to verify the load's integrity. When viewed over an extended period of time, the periodic integrity testing of a load during its idle conditions consumes a significant amount of power, which translates into increased operational cost of the system. Again, this fact is particularly true for temperature control systems which use resistance heating.